Seats and Votes 2012: Evidence from the States

Last week I published a chart showing the historical relationship between House seats and votes since 1940.  Using the district-level level 2012 results compiled by David Wasserman, I plotted the percent of votes and seats for the Democrats in the 33 states where both parties had at least one member of the delegation.  That graph bears a striking similarity to the earlier chart depicting the historical relationship.


If I apply the same method to measure bias as I described here using the “logit” of the seats and votes shares, I get a slope that is nearly identical to what I estimated for the historical relationship.

Slope (β) 1.92 2.08
Intercept (α) 0.034 -0.189
Democratic Seat Bias 3.7 -20.5

However the intercept term, α, shows a much larger bias in the Republican direction than the four-seat pro-Democratic bias we found historically.  In these 32 states where both parties won at least one seat in 2012, the two parties garnered nearly identical numbers of votes, but the Democrats were rewarded with just 176 or 177 of the 388 seats (45.5%) in these states.

In the eighteen states with homogeneous delegations, the Democrats netted only one seat.  They swept all fourteen seats in Massachusetts and Connecticut and picked up ten more in six other states. The Republicans scored best in Oklahoma and Arkansas, but Massachusetts alone has as many seats (nine) as those two states combined.  All told the Republicans won twenty-three seats in ten of these homogeneous states.  The Democrats’ one-seat gain the homogenous states left them facing an estimated 19-20 seat deficit after adjusting for the votes they won.